THE EARTHWORM 



37 



It now becomes elongated antero-posteriorly and flattened 

 from above downwards, so as to form an oval plate with 



Fig. 8 



Segmentation and early stages of development of Lumbricus, after E. B. Wilson. 

 A, />, C, D, successive stages of segmentation. E. Blastula stage. F. Commence- 

 ment of invagination ; the macromeres form a flat plate on the ventral side. G. An 

 embryo somewhat younger than F viewed from above, showing the mesomeres and 

 mesoblast rows derived from them. //. Gastrula stage viewed from below, showing the 

 wide oval blastopore bounded by mncrumeres ; at the sides the micromeres are growing 

 over the macromeres. J. Later stage, showing the elongated blastopore and the further 

 overgrowth of the macromeres by the micromeres. K, Optical longitudinal section 

 through a later stage after closure of the blastopore. bp, blastopore ; ec, ectoderm ; 

 en, endoderm ; ent, enteron ; mac, macromeres ; ines, mesoblast ; uric, micromeres ; mm, 

 mesomeres. 



rounded edges. The cells on one side, the lower side, are 

 larger than those on the upper side, and become tall and 



